A typical semiconductor IC for use in a power device (referred to as power device semiconductor IC) has an over-current detection function as shown in FIG. 5. Such power device semiconductor IC is used in power circuits and output circuits of different kinds of apparatuses. A power device semiconductor IC is designed to detect over-current that flows in a power device and, if over-current is detected, limit the current to protect the internal as well as external circuitries from being damaged by the over-current. The power device semiconductor IC is required to provide operational security and several sophisticated operational features including: operational accuracy; dormancy under normal operating conditions; and capability of protective operation when over-current exceeds a predetermined level. In addition, the power device semiconductor IC is preferred to be inexpensive as other electronic components.
The semiconductor IC shown in FIG. 5 has a current input terminal 52 and a current output terminal 53 respectively connected to the collector and the emitter of a power NPN transistor 54. The base of the power NPN transistor 54 is controlled by an NPN Tr drive circuit 55. When for example the power NPN transistor 54 and the NPN Tr drive circuit 55 are used in an output circuit, they amplify the signal input into the NPN Tr drive circuit 55 and provide at the current output terminal 53 stable amplified power that is constant in voltage or current.
The magnitude of the current output from the emitter of the power NPN transistor 54 to the current output terminal 53 is substantially equal to that of the current input to the collector of the power NPN transistor 54 via the current input terminal 52. Over-current detection functionality for limiting this current within a predetermined range can be attained by a resistor 56 and a comparator 57. The resistor 56 is provided to determine the magnitude of the current that flows from the current input terminal 52 to the collector of the power NPN transistor 54. The resistor 56 generates a voltage drop in accord with the magnitude of the current.
The voltage across the power NPN transistor 54 increases with the collector current of the power NPN transistor 54. When this voltage exceeds the threshold voltage of the comparator 57, the output of the comparator 57 appearing at the output terminal 58 thereof is pulled up from a low level to a high level. This change in the output level is transmitted to the NPN Tr drive circuit 55 to disable the power NPN transistor 54.
In order to improve the accuracy of current detection with the circuit of FIG. 5, it is necessary to accurately or correctly measure the magnitude of current to be detected. Since the current that can be passed through one probe of a tester is limited in magnitude to a small value, it is impossible in a wafer test to directly detect or measure a large over-current.
Then, it is necessary in the wafer test to measure the resistance of (or the voltage drop across) the resistor 56 for a permissible current through the probe and compare the measured voltage drop with the threshold of the comparator 57 to determine the maximum permissible level of over-current. To measure the resistance of the resistor 56, source electrodes 61 and 64 and major electrodes 62 and 63 are formed on the chip of the semiconductor IC. Then the small resistance of the resistor 56 is measured as accurately as possible by flowing current between the source electrodes 61 and 64. This can be done by measuring the voltage between the measurement electrodes 62 and 63; In actuality, however, the threshold voltage of the comparator 57 cannot be accurately measured, since the resistor 56 has only a small resistance and is connected between the input terminals of the comparator 57.
Means for improved accurate measurement of the threshold voltage of the comparator 57 has been disclosed in Japanese Patent Application Laid Open 2001-53120, in which electric connection means such as a Zener diode is provided for cutting off the electric connection between the current detection resistor 56 and one input end of the comparator 57 during a wafer test on over-current detection functionality and for re-establishing the electric connection after the test.
However, the disclosed prior art requires not only an electric connection means such as a Zener diode used only in the wafer test but also two extra terminals for feeding high-voltage pulses to the electric connection means. As a consequence, a semiconductor IC having current detection functionality has a complicated structure and must be structured to withstand high-voltage pulses, which increases the cost of the IC.